Méndez has been a member of several other bands, including Fifth to Infinity, Proxima and Vinterkrig. Méndez joined Opeth right before the recording of their third album My Arms, Your Hearse began. However, time conflicts such as joining too late kept him from playing on the album, so the bass lines were recorded by Opeth frontman Mikael Åkerfeldt. Martín then learned the bass parts and played bass for the few subsequent live dates done for the album. His first real recording with Opeth was their fourth studio album, Still Life. He has been with the band ever since.

Méndez is popular among Opeth fans for his silent stage presence (with the slight exception of some South American gigs where he has spoken in his native language). During Opeth performances Méndez rarely speaks or demonstrates specific emotion, although he has a 'no-no' or 'windmill' style of headbanging– shaking his head back and forth or (counter-)clockwise– a sort of mutual affinity among fans and band alike, something Åkerfeldt often joked about on stage before Méndez had his hair cut. He has since grown out his hair as of 2011, and sometimes headbangs in this style again, to great applause from the crowd.